Storage Tank For A Cryogenic Fluid With A Partitioned Cryogen Space

ABSTRACT

A cryogenic storage tank comprises a partition that divides a cryogen space into a main storage space and an auxiliary space. A valve disposed inside the cryogen space is associated with a first fluid passage through the partition. The valve comprises a valve member that is actuatable by fluid forces within the cryogen space. A second fluid passage through the partition comprises a restricted flow area that is dimensioned to have a cross-sectional flow area that is smaller than that of a fill conduit such that there is a detectable increase in back-pressure when the main storage space is filled with liquefied gas.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/681,755 filed Mar. 2, 2007, entitled “Storage Tank For A CryogenicFluid With A Partitioned Cryogen Space”. The '755 application is herebyincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to a storage tank for a cryogenic fluidwith a partitioned cryogen space. More particularly, a partition dividesthe cryogen space into two storage spaces with the apparatus and methodcomprising at least two fluid passages through the partition and atleast one valve that regulates the flow of cryogenic fluid between thetwo storage spaces.

BACKGROUND OF THE INVENTION

Cryogenic fluids comprise liquefied gases that generally have boilingpoints below −100° C. (about −150° F.) at atmospheric pressure. Examplesof cryogenic fluids include liquefied natural gas (LNG), and othergases, such as nitrogen, oxygen, carbon dioxide, methane and hydrogen,that are storable in liquefied form at cryogenic temperatures.

To prevent cryogenic fluids from boiling off and to increase the timethat they can be stored in liquefied form, cryogenic fluids can bestored in thermally insulated storage tanks that consist of an innerstorage vessel mounted within an outer shell, with thermal insulationprovided by insulating materials and a vacuum disposed in the spacebetween the inner vessel and the outer shell. The inner vessel defines acryogen space in which a liquefied gas can be stored at cryogenictemperatures. Such an arrangement reduces the transfer of heat from theambient environment to the cryogenic fluid stored within the cryogenspace, but some heat transfer into the cryogen space, which can also bereferred to as “heat leak” is inevitable. Heat leak warms the cryogenicfluid, which lowers the density of the liquefied gas and increases thebulk temperature and pressure of the cryogenic fluid. In a fixed volume,the lowered density of the liquefied gas causes an increase in thedensity of the cryogenic vapor and some vapor will be condensed backinto the expanding liquid. Overall, the volume of liquefied gas in thecryogen space will increase steadily as the bulk temperature and vaporpressure increase due to heat leak. If the overall pressure in thecryogen space rises above the set point of the pressure relief valve,vapor is vented from the cryogen space to atmosphere, or to a recoverysystem or directly to an end user. For example, with what is known inthe industry as an economizer system, vented vapor can be delivereddirectly to an engine. However, it is still preferable to reduce ventingcryogenic fluid from the cryogen space, and so it is desirable to designstorage tanks to reduce heat leak so that cryogenic fluids can be storedfor longer periods of time without venting. Each pipe that penetratesthrough the insulating space and into the cryogen space provides athermal conduction path that can contribute to heat leak. Reducing thenumber of pipes that extend between the inner storage vessel and theouter shell can reduce heat leak. Heat leak can also be reduced byselecting materials with lower thermal conductivity for the structuralsupports for the inner storage vessel.

Another method of increasing holding times and reducing the possibilityof venting vapor from the cryogen space is to reserve a portion of thecryogen space for vapor when the storage tank is filled. This vaporspace is known as the ullage space, and the ullage space provides avolume for cryogenic fluid to expand into so that the tank does notbecome liquid full before reaching the relief valve set point pressure.If a storage tank is filled completely with a liquefied gas, withoutreserving a vapor-filled ullage space, even a very small amount of heatleak can result in a rapid increase in storage pressure, because thereis little space into which the liquefied gas can expand. Accordingly, itis common practice when filling a storage tank to reserve an ullagespace that is not filled with liquefied gas. To assist with preventingan ullage space from being filled with cryogenic liquid while filling astorage tank, U.S. Pat. No. 5,404,918, entitled, “Cryogenic LiquidStorage Tank” (the '918 patent), discloses a storage tank with apartitioned cryogen space with only one passage means between the maintank and the ullage space. The cross-sectional flow area of the passagemeans is smaller than the cross-sectional flow area of the fill line sothat the liquefied gas is restricted from flowing into the partitionedullage space when the cryogen space is being filled. A problem with thisarrangement is that although the flow area of the passage means issmaller than the cross-sectional flow area of the fill line, it canstill allow a quantity of liquefied gas to flow into the ullage spaceduring filling, especially when the passage means is located near thebottom of the partition to assist with draining liquefied gas back intothe main tank. Perhaps more importantly, another problem with thisarrangement is that the flow restriction caused by the passage meansalso acts to restrict flow of the cryogenic fluid from the ullage spaceback into the main tank. When a storage tank is being filled, if thestorage tank is not initially empty there can be some liquefied gasalready inside the ullage space. This is common for storage tanks usedto carry fuel for a vehicle engine because completely emptying thestorage tank will leave the vehicle out of fuel and stranded. Whenre-filling a partially empty storage tank, the newly introducedliquefied gas flowing into the main tank during the re-filling processcan condense the vapor inside the main tank thereby lowering thepressure in the main tank. Meanwhile, the flow restriction provided bythe passage means restricts the flow rate of liquefied gas that isflowing from the ullage space back into the main tank, driven under suchcircumstances by the pressure in the ullage space being higher than thepressure in the main tank. At the end of the filling process, therestricted flow of liquefied gas from the ullage space can result in asignificant amount of liquefied gas being trapped inside the ullagespace, resulting in a reduced volume reserved for vapor. This outcome isdisadvantageous because a reduced volume of vapor at the end of there-filling process means that there is less volume for cryogenic fluidto expand into, which in turn means shorter hold times before vapor isvented from the cryogen space. Also, during use after re-filling, if theliquefied gas is being delivered from the storage tank at a high rate,another disadvantage of storage tanks with one restricted fluid passagethrough the partition is that the flow restriction provided by thesingle fluid passage can slow down the rate at which the liquefied gascan be delivered. Some, of the problems associated with the designtaught by the '918 patent are overcome by the design taught by U.S. Pat.No. 6,128,908 entitled “Cryogenic Liquid Storage Tank with IntegralUllage Tank” (the '908 patent). The '908 patent teaches an arrangementsimilar to that of the '918 patent except that the passage between theullage tank and the main tank connects the lower part of the ullage tankwith the upper part of the main tank. This prevents liquefied gas fromflowing into the ullage tank during the first part of the fillingprocess, but the small diameter of the provided passage still restrictsthe rate at which liquefied gas can flow back into the main tank.However, this design introduces new problems because it relies upon apressure differential between the ullage tank and the main tank toremove liquefied gas from the ullage tank and this can also result inliquefied gas being trapped inside the ullage tank. The effectiveness ofthe storage tank taught by the '908 patent can be compromised if heattransfer through the partition wall between the main tank and the ullagetank cools the vapor in the ullage tank to such a degree that itcondenses vapor in the ullage space thereby reducing vapor pressuretherein. If the pressure differential is not high enough to effectivelyremove the liquefied gas from the ullage space through the verticalpassage, when the tank is re-filled this can result in the reservedvapor space being smaller than desired. In addition, anotherdisadvantage of the design taught by the '908 patent is that, because ofthe small diameter of the vertical passage between the ullage space andmain tank, it appears that blockage of the passage is possible, leadingthe developers of this design to introduce into preferred embodiments apressure relief device operative to prevent over-pressurization of theullage space, adding to the cost of manufacture, as well as adding tothe possible failure modes should the pressure relief valve fail.

U.S. Pat. No. 5,685,159 entitled, “Method and System for Storing ColdLiquid” (the '159 patent), discloses an arrangement that comprises amain tank and an auxiliary tank, which can act as an ullage space forthe main tank. In a disclosed preferred embodiment, when the auxiliarytank is external to the main storage tank, the system can comprise asolenoid valve that controls flow between the main storage tank and theauxiliary tank. An electronic controller can be programmed toautomatically actuate the solenoid to close the valve when the tank isbeing filled and to otherwise keep the valve open to allow the auxiliarytank to communicate with the main storage tank and to act as the ullagespace. A disadvantage of this arrangement is that the conduit betweenthe main tank and the auxiliary tank introduces another heat transferpath into the cryogen space. However for maintenance purposes thesolenoid valve should be located outside of the storage vessel where itcan be accessed for servicing and where the solenoid will not itselfintroduce heat into the cryogen space.

The '159 patent also discloses other preferred embodiments in which theauxiliary tank is located inside the main storage tank. In theseembodiments, instead of a valve, capillary tubes are employed torestrict flow of cryogenic fluid into the auxiliary tank during fillingand to allow communication between the auxiliary tank and the mainstorage tank so that the internal auxiliary tank can act as the ullagespace and so that cryogenic fluid (liquid and/or vapor) can drain backinto the main storage tank. One problem with this arrangement is that atleast one capillary tube is preferably associated with the low point ofthe ullage space so that liquefied gas that enters or condenses insidethe ullage space can be drained there from through the capillary tubethat is associated with the low point of the ullage space. Since thecapillary tubes remain open, they can allow flow of the cryogenic fluidinto the auxiliary tank during re-filling, which can result in reduced areduced volume of vapor space.

SUMMARY OF THE INVENTION

A cryogenic storage tank comprises a cryogen space defined by athermally insulated vessel for storing a cryogenic fluid and a partitiondividing the cryogen space into a main storage space and an auxiliaryspace. In a preferred embodiment, a conduit connects the main storagespace with a coupling outside the storage tank, a valve is disposedinside the cryogen space and associated with a first fluid passagethrough the partition, the valve comprising a valve member that isactuatable by fluid forces within the cryogen space to move between anopen position and a seated position, and a second fluid passage isprovided through the partition, wherein the second fluid passagecomprises a restricted flow area that is dimensioned to have across-sectional flow area that is smaller than that of the conduit suchthat there is a detectable increase in back-pressure in the conduit whenthe cryogenic fluid is being introduced into the main storage space, andthe main storage space is filled with liquefied gas.

In preferred embodiments the valve member is movable from the openposition to the seated position when fluid pressure within the mainstorage space is higher than fluid pressure within the auxiliary spaceby a predetermined amount. For example, to move the valve member to theseated position, the predetermined amount by which the fluid pressurewithin the main storage space must exceed the fluid pressure within theauxiliary space is fixed by a bias that the valve member has to the openposition. For example, the valve member can be oriented within thecryogen space so that gravitational forces add to the fluid forces thatact on the valve member, wherein the gravitational forces bias the valvemember towards the open position. In such embodiments, the valve membercan be moveable along a vertical or sloped axis between the open andseated positions.

In preferred embodiments, the fluid forces acting on the valve membercomprise static liquefied gas pressure and vapor pressure, and when thevalve is open, dynamic fluid momentum forces caused by cryogenic fluidthat flows through the valve. Static liquefied gas pressure isdetermined by the depth of the liquefied gas at the point where thefluid force is measured, this force being greatest at the lowest pointwithin the cryogen space, and zero above the level of the liquefied gas.

In a preferred embodiment the valve member is spherical in shape.Persons familiar with the technology involved here will understand thatother shapes are possible. An advantage of a spherical shape is that itis relatively easy to manufacture and it is not necessary to guide themovement and orientation of the valve member for it to seat properly inthe seated position. This preferred arrangement simplifies the valvemechanism, improving the robustness of the valve, which is desirablesince the valve is installed in a location that is difficult to accessfor repair. The simple design also reduces manufacturing and assemblycosts. The valve member can be made from materials selected from thegroup consisting of metallic materials and ceramic materials. Thesematerials are suitable for use in a cryogenic environment. Whilemetallic materials such as stainless steel have been tested in aprototype cryogenic storage tank, persons familiar with the technologyinvolved here will understand that other suitable materials known to bedurable in a cryogenic environment could be substituted, such as othermetallic materials like carbon steel, or non-metallic materials such aspolytetrafluoroethylene or glass.

In preferred embodiments the first fluid passage and the valve are sizedto allow the cryogenic fluid to flow from the auxiliary space to themain storage space at a rate that keeps the liquefied gas at a levelwithin the auxiliary space that is substantially the same as the levelof the liquefied gas within the main storage space when the cryogenicfluid is being dispensed from the cryogen space. This allows the levelof the liquefied gas to be maintained at about the same level on bothsides of the partition when liquefied gas is being dispensed from thestorage tank. The disclosed storage tank provides an advantage overprior art designs that employed a single restricted fluid passagethrough the partition, because there could be conditions under which thedelivery of liquefied gas from the main storage space of a prior artstorage tank is constrained by the rate at which liquefied gas can bedrained from the auxiliary space. In one embodiment, the storage tankcomprises an outlet conduit extending from the main storage space tooutside the thermally insulated vessel, wherein the first fluid passageand the valve, when open, each respectively have a cross-sectional flowarea that at its smallest section is at least as large as thecross-sectional flow area of the outlet conduit at its smallest section.Sizing the fluid passage and valve in this way ensures that delivery ofthe cryogenic fluid from the cryogen space is not constrained by therate at which fluid can be transferred from the auxiliary space to themain storage space.

In some embodiments the valve member is movable to the open positionwhen vapor pressure within the auxiliary space is equal to vaporpressure within the main storage space. In this case, this means thatthe valve member is biased to the open position. For example, a valvemember can be urged to the closed position at a time when the mainstorage space is being filled with cryogenic fluid. After the re-fillingprocess is completed, because of the opening through the second fluidpassage and heat transfer through the partition, eventually vaporpressures become substantially equalized, and with such an embodiment,this allows the valve to open so that the level of liquefied gas canalso equalize.

The partition can be oriented to extend between an upper region and alower region of the cryogen space, thereby defining one side of thecryogen space as the auxiliary space. A partition that is substantiallyperpendicular to the longitudinal axis, defining the auxiliary space asone end of the cryogen space is a preferred embodiment because it isrelatively easy to build a storage tank with this configuration.However, because this results in liquefied gas occupying the cryogenspace on both sides of the partition after equilibrium conditions arereached after venting and re-filling, it can be advantageous for thesecond fluid passage to comprise a conduit with a first opening into themain storage space near the upper region of the cryogen space, a secondopening into the auxiliary space near the lower region of the cryogenspace. An advantage of this arrangement is that liquefied gas can bedrained through both first and second fluid passages when there is asufficient pressure differential during venting and re-filling becausethe second fluid passage opens into the lower region of the auxiliaryspace. This arrangement also helps to maintain a pressure differentialduring venting and filling because as long as the open end in theauxiliary space is submerged in liquefied gas, vapor can not escapethrough the second fluid passage.

Nevertheless, simpler embodiments of the second fluid passage are alsoacceptable and can still function according to the disclosed method. Forexample, it is simpler and less expensive for the second fluid passageto be a hole through the partition, whereby the second fluid passage hasa length that is defined only by the thickness of the partition. Insteadof a simple hole, the second fluid passage can comprise a plate with anorifice that defines the restricted flow area in the second fluidpassage. The orifice plate adds to the length of the second fluidpassage and can facilitate more precise machining of the orifice size.

In yet another embodiment, the second fluid passage can further comprisea valve disposed therein, this being the second valve disposed withinthe cryogen space with the valve associated with the first fluid passagebeing the first valve. Like the first valve, the second valve cancomprise an associated valve member that is actuatable by fluid forceswithin the cryogen space to move between an open position and a seatedposition. In a preferred arrangement for this embodiment, the valvemember associated with the second valve is movable by differential fluidpressure, to move the valve member to the open position to allowcryogenic fluid to flow from the main storage space to the auxiliaryspace when fluid pressure inside the main storage space is higher thanfluid pressure inside the auxiliary space by a predetermined amount.Closing forces acting on the valve member of the second valve can urgethe valve member to the seated position when fluid pressure within themain storage space does not exceed fluid pressure within the auxiliaryspace by a predetermined amount. In such embodiments the second valve isbiased in the closed position so that fluid pressure in the main storagespace must exceed fluid pressure in the auxiliary space by apredetermined amount to open the second valve. This is advantageousduring venting and re-filling because it prevents vapor from escapingthe auxiliary space through the second fluid passage, while alsopreventing newly introduced cryogenic fluid from flowing from the mainstorage space to the auxiliary space, in which an ullage volume is beingpreserved. Later, after re-filling is completed, the second valve canopen to allow vapor to expand into the auxiliary space if liquefied gasexpands and fluid pressure rises within the main storage space. In apreferred embodiment of the second valve, it is oriented within thecryogen space so that the valve member associated with the second valveis gravity biased in the seated position. The term “seated” position isused here and elsewhere in this specification because the first andsecond valves remain operative even if a perfectly fluid-tight seal isnot formed when the valve member is in the seated position. That is,whether seated or perfectly sealed, when urged against the valve seat,the valve member will restrict flow through the associated fluid passageto a degree that allows the apparatus to operate in accordance with thedisclosed method. Like the first valve, in preferred embodiments of thesecond valve, the valve member associated with the second valve isspherical, and the valve member can be made from materials selected fromthe group consisting of metallic materials and ceramic materials.

In one embodiment the second valve permits fluid flow through the secondfluid passage only in a direction from the main storage space to theauxiliary space. This helps to maintain a pressure differential duringventing and re-filling but prevents liquefied gas from being drainedfrom the auxiliary space through the second fluid passage, as ispossible when the second fluid passage comprises a conduit extendingfrom an upper region of the main storage space to a lower region of theauxiliary space.

In preferred embodiments the auxiliary space is smaller than the mainstorage space. For example, for a storage tank with a cryogen space thatis about 75 U.S. gallons (about 0.28 cubic meters), the auxiliary spaceis preferably less than 15% by volume of the cryogen space, and in someembodiments can be less than 10% by volume of the cryogen space.

In some embodiments, the storage tank comprises an outlet conduitextending from the main storage space to outside the thermally insulatedvessel. In other embodiments, the storage tank comprises a pump with aninlet disposed within the main storage space with an outlet conduitextending from the pump to outside the thermally insulated vessel. Astorage tank with a pump is more suitable for storage tanks that areemployed to deliver a cryogenic fluid at a higher pressure, whereas anoutlet conduit without a pump can be employed when the cryogenic fluidis delivered at a lower pressure, with the cryogenic fluid pushed fromthe cryogen space by the vapor pressure therein. A storage tank with anoutlet conduit can also be combined with a system with an external pump.

A vent conduit is associated with the storage tank for venting vaporfrom the cryogen space. In preferred embodiments a portion of the fillconduit can also serve as part of the vent conduit. In one sucharrangement the vent conduit branches off from the fill conduit outsidethe thermally insulated vessel and valves associated with the fillconduit and the vent conduit are operative so that vapor can be ventedthrough the vent conduit or through the fill conduit and fill stationhose to pre-cool the fill station hose prior to re-filling.

A method is provided of holding a cryogenic fluid comprising liquefiedgas and vapor in the disclosed storage tank. The method comprisesestablishing a vapor-filled ullage space when the main storage space isbeing vented and re-filled with cryogenic fluid, by draining liquefiedgas from the auxiliary space to the main storage space through a firstfluid passage provided through the partition. The first fluid passage issized to allow the liquefied gas to flow there through under theinfluence of a vapor pressure differential permitted by the partition.The liquefied gas drains at a rate that allows the auxiliary space to bedrained of liquefied gas to a level that establishes the vapor-filledullage space with a volume that is at least as large as a predeterminedullage volume in less than the time to vent and re-fill the main storagespace with cryogenic fluid. The method further comprises preserving thevapor-filled ullage space by restricting flow of liquefied gas throughthe first fluid passage from the main storage space into the auxiliaryspace when the cryogen space is being re-filled with cryogenic fluid andrestricting cryogenic fluid flow through a second fluid passage. Themethod further comprises stopping the flow of cryogenic fluid into thecryogen space through the fill conduit when detecting an increase inback-pressure that occurs when the main storage space is filled withliquefied gas.

The method can further comprise transferring the cryogenic fluid betweena lower region of the auxiliary space and an upper region of the mainstorage space, when the cryogenic fluid is flowing through the secondfluid passage.

In a preferred embodiment, the step of restricting flow through thefirst fluid passage comprises regulating flow there through with a valvethat comprises a valve member that is actuatable between an open and aseated position by fluid forces within the cryogen space, whereby fluidflow is restricted by the valve when the valve member is in the seatedposition. In one embodiment, the valve member is urged towards theseated position when fluid pressure acting on the valve member fromfluid within the main storage space is higher than fluid pressure actingon the valve member from fluid within the auxiliary space by apredetermined margin, and the valve member is otherwise urged to theopen position.

When re-filling the main storage space with cryogenic fluid, the methodcan further comprise introducing the cryogenic fluid at a flow rate thatcauses cryogenic fluid level within the main storage to increase at afaster rate than cryogenic fluid level can rise within the auxiliaryspace, whereby static fluid pressure and fluid momentum pressure actingon the valve member urge it towards the seated position.

In addition to detecting an increase in back-pressure, the step ofdetecting when the storage tank is full can further comprise processinga signal from a level sensor to determine the level of liquefied gaswithin the main storage space.

As already described in describing the preferred embodiments of theapparatus, the method can comprise venting vapor from the main storagespace through the fill conduit prior to re-filling the storage tank withcryogenic fluid. This reduces the number of conduits that extend betweenthe outside environment and the cryogen space, helping to reduce heatleak into the cryogen space. This arrangement also allows the method tofurther comprise venting vapor through the fill station hose to pre-coolit prior to delivering cryogenic fluid to the storage tank. Vapor ventedthrough the fill hose can be directed to the fill station where it canbe condensed and re-liquefied. In other circumstances the method cancomprise venting vapor from the main storage space through a vent pipeprior to re-filling the storage tank with cryogenic fluid, for exampleif the fill station is not equipped or unwilling to receive vented vaporfrom the storage tank.

When re-filling the main storage space, the method can further comprisenot introducing cryogenic fluid into the cryogen space until vaporpressure within the main storage space is below a predetermined value.

In embodiments of the apparatus that comprise a second valve associatedwith the second fluid passage the method can further comprise regulatingfluid flow through the second fluid passage by using fluid forces withinthe cryogen space to actuate a valve member, whereby the second valveopens to allow cryogenic fluid to flow from the main storage space tothe auxiliary space when vapor pressure in the main storage space ishigher than vapor pressure within the auxiliary space by a predeterminedamount. If the second valve is a one-way valve, and the method canfurther comprise allowing fluid to flow through the second valve onlyfrom the main storage space to the auxiliary storage space.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a schematic illustration of a preferred embodiment of astorage tank with a partitioned cryogen space. FIGS. 1A through 1G areenlarged views of the fluid passages through the partition with each ofthese enlarged views showing a different embodiment of the same generaldesign concept. The partition divides the cryogen space into a mainstorage space and an auxiliary space. Each of the illustratedembodiments comprises a valve associated with a first fluid passagelocated near the bottom of the partition that allows flow of cryogenicfluid between the auxiliary space and the main storage space and asecond fluid passage with one end that opens into the main storage spacenear the top of the cryogen space. In the embodiments shown in FIGS. 1,1C and 1D the second fluid passage remains open, with the size of theopening restricting flow there through. FIGS. 1E through 1G showembodiments of the second fluid passage that further comprise a valvethat is operable to allow cryogenic fluid to flow from the main storagespace to the auxiliary space and that can restrict flow in the oppositedirection.

FIG. 2A is a plot of cryogenic fluid pressure overlaid with the level ofliquefied gas within the cryogen space on each side of the partition,when the cryogen space is being vented to ready the storage tank forfilling. FIG. 2B is a plot of the same parameters plotted against timefor a different scenario when the main storage space is being re-filled.FIGS. 2A and 2B demonstrate by way of example how the disclosedembodiments are operative to drain liquefied gas from the auxiliaryspace and preserve a desired volume of vapor space at the end of there-filling process, even if the level of liquefied gas in the auxiliaryspace is initially high.

FIG. 3 is a schematic illustration of another preferred embodiment of astorage tank with a partitioned cryogen space. In this embodiment a pumpis disposed within the cryogen space to deliver cryogenic fluidtherefrom.

FIG. 4 is a schematic illustration of yet another embodiment in whichthe partition separates an upper region of the cryogen space from themain storage space that occupies the lower region of the cryogen space.This third embodiment also comprises a valve that allows liquefied gasto flow from the auxiliary space to the main storage space through afirst fluid passage that is associated with the bottom of an auxiliaryspace and a second fluid passage comprises an opening into the auxiliaryspace near the top of the main storage space to allow vapor to flow fromthe main storage space into the auxiliary space.

In the Figures and the detailed description of the preferredembodiments, like reference numbers are used for like components indifferent embodiments. The embodiments are shown schematically and notto scale, with some elements exaggerated or shown symbolically tosimplify and/or more clearly illustrate the embodiments shown.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

With reference to the Figures, storage tanks 100, 300 and 400 areexamples of embodiments that are particularly suited for holdingcryogenic fluids inside thermally insulated vessel 101, which definesrespective cryogen spaces 102 and 402. FIGS. 1, 3 and 4 show that thepresent design can be applied to storage tanks with differentconfigurations for the partition, and for storage tanks that are usedfor delivering the stored fluid at lower or relatively high pressures.For example, the embodiment of FIG. 3 shows a storage tank with aninternal pump that can be used to deliver the stored fluid at a higherpressure. The different illustrated embodiments show that the presentdesign can be useful for many different applications where such storagetanks are employed for holding and dispensing liquefied gases. Forexample, gases can be stored and dispensed in liquefied form to providerefrigeration for industrial processes or if the gas is used in thegaseous phase it can be stored in liquefied form to increase the storagedensity. A particularly suitable application for the disclosed storagetanks is for holding and dispensing a liquefied gas such as natural gas,pure methane, hydrogen, mixtures thereof, or other like combustiblegases that can be delivered as fuel to an engine. The engine can beused, for example, to power a vehicle, drive other machinery, or togenerate electrical power.

However, because cryogenic fluids are kept inside the storage tank attemperatures that are much lower than ambient temperatures, heat leakinto the cryogen space can convert some of the stored liquefied gas intovapor, leading to increases in the vapor pressure. If the vapor pressureincreases to a predetermined threshold, a pressure relief valvetypically opens to vent some of the vapor and reduce the vapor pressurethereby preventing over-pressurization and damage to the storage tank.However, this results in some of the stored fluid being wasted andreleased into the surrounding atmosphere unless the vent is connected toa recovery system, which can add more cost and complexity to the system.Except for wasting the stored cryogenic fluid, if the stored fluid is asubstance that is harmless to the environment, such as liquefied oxygenor nitrogen, there may not be concerns with venting such substances toatmosphere through a vent stack or in a well-ventilated location. Forother substances such as natural gas, venting to atmosphere is notdesirable, however, a recovery system may not be practical if thedispensed gas is used in liquefied form and there is no immediate usefor the vented vapor, or if the end user normally uses the dispensed gasat a higher pressure than the relief pressure for the storage tank andit is not practical to boost the pressure of the vented vapor to therequisite pressure. While a pressure relief system remains a safetyfeature to guard against over-pressurization of the storage tank, anobjective of the present design is to provide improved features thatreduce the frequency or likelihood of having to vent vapor from cryogenspace 102 when the storage tank is in normal operation, holding anddispensing liquefied gas.

Storage tanks 100, 300 and 400 all comprise a partitioned storage spacethat helps to reserve some of cryogen space 102 from being filled withliquefied gas when it is being filled so that cryogenic fluid can expandwithout rapidly increasing fluid pressure. As already discussed, the useof a partition in a cryogen space for this purpose is already known topersons familiar with the technology involved here, but the presentdesign makes improvements over the previously known arrangements byautomatically regulating flow between the spaces on either side of thepartition depending upon the circumstances at the time, for example, attimes when the cryogen space is being vented prior to re-filling thedisclosed arrangement facilitates the draining of liquefied gas fromauxiliary space 108, or during the re-filling process liquefied gas cancontinue to drain from auxiliary space 108 while a vapor space isreserved therein, or in the course of holding and dispensing liquefiedgas, vapor in main storage space 106 can flow into auxiliary space 108and the level of liquefied gas can equalize in cryogen space 102.

With reference to the embodiments illustrated in the Figures, partitions104 and 404, in the respective embodiments, divide cryogen space 102,402 into main storage space 106, 406 and smaller auxiliary storage space108, 408. Cryogen space 102 is filled by introducing cryogenic fluidthrough conduit 110, which comprises open end 111 disposed in mainstorage space 106. Cryogenic fluid can be introduced into conduit 110through coupling 114, which is located outside storage tank 100, 300 or400. Coupling 114 can comprise an integral shut off valve that openswhen coupled to a fill nozzle, and closes when a fill nozzle is notcoupled to coupling 114. Manifold block 115 can be used to reduce thenumber of fittings and connections to simplify assembly and reduce thenumber of potential leak points. In FIG. 1, internal passages withinmanifold block 115 connect the conduits that extend into the interior ofvessel 101 with three outside connections. As already implied, oneinternal passage leads from conduit 110 to coupling 114 for fillingcryogen space with cryogenic fluid. Optional shut off valve 116 can beinstalled between manifold block 115 and coupling 114 and conduit 110,or to achieve the same effect with less connections, the valve assemblycan be installed into a bore made in manifold block 115, wherebymanifold block 115 then services as the body for shut off valve 116.With the illustrated embodiment, vapor can be vented from main storagespace 106 through conduit 110. This is different from conventionalsystems, which commonly vent vapor to atmosphere, resulting in the gasbeing wasted and released into the environment. A preferred method isdisclosed by co-owned Canadian Patent No. 2,505,606, entitled, “StorageTank for Cryogenic Liquid and Method of Re-Filling Same” (the '606patent). The '606 patent discloses an apparatus and method for ventingthrough the same conduit that can be used to re-fill the storage tank.Vapor that is vented through coupling 114 can be used to pre-cool thehose and/or piping that is used to deliver the liquefied gas to cryogenspace 102, and this vented vapor can be re-condensed by the fillingstation and thereby recovered. A second internal passage within manifoldblock 115 can be employed to direct vented vapor from conduit 110 toconduit 117. Pressure relief valve 118 can be installed in manifoldblock 115 or on conduit 117 downstream from manifold block 115. If thefilling station does not wish to accept vapor that is vented fromcryogen space 102, the vapor can be vented through conduit 117. When thestorage tank is in normal operation away from the filling station,pressure relief valve 118 automatically opens when vapor pressure insidecryogen space 102 exceeds a predetermined set point to vent vaporthrough conduit 117. In the embodiments shown in FIGS. 1 and 4, a thirdinternal passage connects outlet conduit 112 with delivery conduit 119,through which cryogenic fluid is deliverable to an end user. In theembodiment shown in FIG. 3, outlet conduit 112 can still be optionallyinstalled to connect to drain line 319, but cryogenic pump 350 isdisposed within main storage space 106 to pump most of the liquefied gasto an end user through delivery conduit 352. Compared to the otherembodiments, cryogenic pump 350 makes the embodiment of FIG. 3 moresuitable, for example, for delivering gas to an end user at higherpressures.

Referring now to partition 104, and its associated features. Cryogenicfluid can flow between main storage space 106 and auxiliary space 108through two fluid passages provided through partition 104, namely firstfluid passage 120 and second fluid passage 130. First fluid passage 120is provided near the bottom of cryogen space 102 to facilitate drainingliquefied gas from auxiliary space 108 to main storage space 106, andunder certain conditions, such as when fluid pressure is about equal inthe two spaces, to also allow liquefied gas to flow in the reversedirection, from main storage space 106 to auxiliary space 108, forexample allowing the level of the liquefied gas to equalize on bothsides of partition 104. Two-way flow through first fluid passage 120 isfacilitated when valve 122 is biased in an open position, for examplewhen valve 122 and valve member 124 are oriented so that gravity biasesvalve member 124 to be spaced from valve seat 126, as shown in FIG. 1A.Allowing two-way flow through valve 122 prevents large differences inthe level of liquefied gas on opposite sides of the partition, avoidingan imbalance in the static liquid pressures, which might otherwiserequire the structure of partition 104 to be stronger, adding to theweight and cost of the apparatus.

Valve 122 regulates flow through first fluid passage 120 with valvemember 124 being automatically actuated by fluid forces within cryogenspace 102. That is, valve 122 is not actuated manually, or by anactuator with a driver such as a solenoid or by a pneumatic or hydraulicpiston, or by some other suitable mechanical mechanism. While additionalforces can be used to bias the position of the valve member, it is fluidforces acting on valve member 124 that automatically operate it based onthe circumstances at the time. These fluid forces include vapor pressureand static liquefied gas pressure when valve member 124 is seated. Whenvalve 122 is open and cryogenic fluid is flowing through first fluidpassage 120, the fluid forces acting on valve member 124 further includedynamic fluid momentum forces, acting in the direction of fluid flow. Inthe illustrated preferred embodiments of valve 122, it is oriented sothat gravitational forces also act on the valve member to bias it in theopen position. Screen 128 has large openings so that liquefied gas canflow freely into first fluid passage 120, while screen 128 preventsvalve member 124 from falling out of first fluid passage 120. Instead ofscreen 128, other means such as protrusions from the walls of firstfluid passage 120 can be employed to retain valve member 124 in firstfluid passage 120.

Second fluid passage 130 is provided near the top of cryogen space 102to allow vapor from main storage space 106 to flow into auxiliary space108. Second fluid passage 130 preferably has its opening into mainstorage space 106 in the upper region thereof, to reduce flow ofliquefied gas from main storage space 106 through second fluid passage130 to auxiliary space 108 during filling. In the embodiment of FIG. 1and FIGS. 1A through 1D, second fluid passage 130 remains open but ithas a constricted flow area so that a pressure differential can stilldevelop between main storage space 106 and auxiliary space 108. That is,when main storage space 106 is being vented prior to filling, vapor alsovents from auxiliary space 108 through second fluid passage 130, butbecause of the constricted flow area, vapor flow can not keep up withthe rate at which vapor is being vented from main storage space 106,allowing a pressure differential to develop, which helps to pushliquefied gas from auxiliary space 108 to main storage space 106 throughfirst fluid passage 120. Another benefit of the constriction in secondfluid passage 130 is that when main storage space is being filled withliquefied gas, the constricted flow area helps to signal when mainstorage space 106 is filled, because even though open second fluidpassage 130 will allow a small amount of liquefied gas to flow intoauxiliary space 108 after the level of liquefied gas reaches the levelof the opening of second fluid passage 130 into main storage space 106,the constricted flow area causes a detectable increase in back pressurewhen main storage space 106 is full. However, during normal operationwhen the storage tank is being used to hold and dispense cryogenicfluid, and heat leak warms some of the liquefied gas in cryogen space102, the constricted flow area provided by second fluid passage 130 ismore than adequate to allow vapor to flow between main storage space 106and auxiliary storage space 108 to maintain an equalized vapor pressureacross partition 104, such that the vapor space reserved at the end ofthe re-filling process forms an ullage space for cryogenic fluid toexpand into, thereby reducing the rate at which vapor pressureincreases, prolonging hold times, and reducing the need to vent vaporfrom cryogen space 102. Prior art storage tanks that employ only onefluid passage through a partition need to compromise between the need toprovide a constricted fluid passage to prevent the auxiliary space frombeing filled when the main storage space is begin filled, and the needto provide an opening large enough to allow liquefied gas to be drainedfrom the auxiliary space during venting and to allow adequate flowduring normal operation. This drawback of such prior art designs isovercome by the present design.

FIGS. 1A through 1G are enlarged views of partition 104 and the fluidpassages for regulating fluid flow there through, with each Figureshowing by way of example, different arrangements that all share thesame design concept.

With reference to FIG. 1A, this is an enlarged view of the arrangementfor first fluid passage 120 that is shown in FIG. 1. First fluid passage120 comprises a down-turned elbow with the vertical section having anopen end facing towards the bottom of main storage space 106. Valve 122and valve member 124 are oriented so that valve member 124 is gravitybiased in the open position, spaced apart from valve seat 126 as shown.Screen 128 keeps valve member 124 from falling out of first fluidpassage 120.

In FIG. 1B, first fluid passage 120 is in the shape of an upturned90-degree elbow with the upturned open end being in auxiliary space 108.Valve 122B is associated with the vertically aligned portion of theelbow so that valve member 124B is urged by gravity to the open positionaway from valve seat 126B. Screen 128B keeps valve member 124B fromresting against the walls of first fluid passage 120 where it mightotherwise restrict flow there through when valve member 124B is spacedapart from valve seat 126B.

The embodiments of FIGS. 1A and 1B can be paired with an open butconstricted second fluid passage such as second fluid passage 130 shownin FIG. 1 or with second fluid passage 130C shown in FIG. 1C, or with asecond fluid passage that further comprises a valve such as theembodiments shown in FIGS. 1E through 1G, which are described below. InFIG. 1, second fluid passage 130 is simply an orifice provided inpartition 104. The orifice can be made by simply drilling a hole inpartition 104 or by installing an orifice plate.

With reference to FIG. 1C, first fluid passage 120 is also elbow-shaped,but down-turned into main storage space 106 at less than 90 degrees fromthe horizontal axis, with valve 122C located on the downward slopingportion so that gravity still urges valve member 124C to the openposition away from valve seat 126C. This example shows that to gravitybias the valve member it need not be vertically oriented and, in fact,any slope can be employed, with shallower slopes employed if it isdesired to reduce the gravity bias and make the valve close under theinfluence of a smaller fluid pressure differential. Screen 128C allowsfluid to pass through the open end while retaining valve member 124Cwithin first fluid passage 120.

FIG. 1C also shows another embodiment of the second fluid passagelabeled with reference number 130C. Second fluid passage 130C comprisesa conduit that has an open end into the upper region of main storagespace 106 and an open end into the lower region of auxiliary space 108.The advantage of this configuration is that when liquefied gas is beingdrained from auxiliary space 108, the liquefied gas can be drainedthrough both the first and second fluid passages and liquefied gas thatfills second fluid passage 130C prevents vapor from flowing through it,helping to maintain a differential vapor pressure. While the embodimentof FIG. 1 has the advantage of being simpler, it allows more vapor toescape from auxiliary space 108 while main storage space 106 is beingvented and this reduces the vapor pressure differential that helps todrain liquefied gas from auxiliary space 108. Even so, the restrictedflow area of second fluid passage 130 in FIG. 1 does allow a pressuredifferential to develop and be sustained to serve the desired purpose ofdraining liquefied gas from auxiliary space 108.

With reference to FIG. 1D, first fluid passage 120 is in the shape of adown-turned 90-degree elbow with the open down-turned open end being inmain storage space 106. Valve 122D is associated with a verticallyaligned portion of the elbow so that valve member 124D is urged bygravity to the open position away from valve seat 126D. Second fluidpassage 130D is similar to the second fluid passage of FIG. 1C becauseit comprises a conduit that extends from an open end associated with theupper region of main storage space 106 to an opening associated with thelower region of auxiliary space 108. However, in this embodiment secondfluid passage 130D is disposed in main storage space 106 and branchesfrom first fluid passage 120 between valve 122D and the opening intoauxiliary space 108. This configuration shows that the scope of thepresent design concept includes embodiments in which the first andsecond fluid passages share one opening through partition 104. The twofluid passages define two different routes through which cryogenic fluidcan flow between main storage space 106 and auxiliary space 108. Firstfluid passage 120 allows two-way flow of cryogenic fluid between thelower region of main storage space 106 and the lower region of auxiliaryspace 108, while second fluid passage 130D provides a constricted flowpath that allows cryogenic fluid to flow between the upper region ofmain storage space 106 and the lower region of auxiliary space 108. Thatis, in this arrangement, the two fluid passages between main storagespace 106 and auxiliary space 108 share the same opening throughpartition 104, while still functioning as separate fluid passages in amanner similar to the fluid passage arrangement shown in FIG. 1C.

FIGS. 1E through 1G all show embodiments of second fluid passage 130that further comprise a valve that controls fluid flow through thisfluid passage. The valves shown in the embodiments of FIGS. 1E and 1Gare both oriented so that respective valve members 134E and 134F aregravity biased in a closed position. The valves in these twoarrangements operate in substantially the same manner. In FIG. 1E, valve132E is disposed in the vertical part of an up-turned elbow that has itsopen end in auxiliary space 108. Valve member 134E is shown in theclosed position, resting against valve seat 136E. Screen 138E preventsvalve member 134E from being pushed out of second fluid passage 130. InFIG. 1F, valve 132F is disposed in the vertical part of a down-turnedelbow that has its open end in main storage space 106. In thisillustration, valve member 134F is shown in the open position, spacedapart from valve seat 136F. Screen 138F allows cryogenic fluid to flowthrough valve 132F by preventing valve member 134F from sealing againstthe walls of second fluid passage 130. With reference to FIGS. 1E and1F, respective valve members 134E and 134F are lifted into the openposition from the closed position when fluid pressure in main storagespace 106 is higher than fluid pressure in auxiliary space 108. That is,as in the other embodiments, vapor at a higher pressure in main storagespace 106 can flow through second fluid passage 130 into auxiliary space108 thereby moderating the effect of fluid pressure increases caused,for example, by heat leak and volumetric expansion of the liquefied gas.In these embodiments, valves 132E and 132F are gravity biased in theclosed position so that when fluid pressure in auxiliary space 108 ishigher than fluid pressure in main storage space 106, the valve willremain closed and the fluid pressure differential can help to pushliquefied gas into main storage space 106 from auxiliary space 108through first fluid passage 120.

FIG. 1G shows yet another embodiment of second fluid passage 130 thatcomprises a valve. This embodiment shows that valve 132G, which isassociated with second fluid passage 130, need not be gravity biased. Inthis embodiment, when the storage tank is level, valve member 134G canmove to an open position spaced apart from valve seat 136G when vaporpressure in main storage space 106 is higher than the vapor pressure inauxiliary space 108 because it does not need to overcome a gravity bias.Even when the storage tank is not level, the forces of gravity will bemuch less than it would be in the embodiments shown in FIGS. 1E and 1F.Second fluid passage 130, like the other embodiments, including thoseshown in FIGS. 1E and 1F, still provides a constricted flow area, sothat when main storage space 106 is being filled with cryogenic fluidthere will be a rise in back-pressure when main storage space is filled,even when valve member 134G is urged to an open position against screen138G. Like the embodiments of FIGS. 1E and 1F, when vapor pressure ishigher in auxiliary space 108 compared to the vapor pressure in mainstorage space 106, valve member 134G will be pushed against valve seat136G so that a high pressure differential can be maintained to help pushliquefied gas from auxiliary space 108 to main storage space 106 whenmain storage space 106 is being vented or filled.

For the valves associated with the fluid passages through the partitionthat are shown in the Figures, the following equations summarize theiroperation, where:

-   -   P_(m) is the sum of the fluid pressure forces acting on the        valve member from the main storage space side, including the        vapor pressure and static liquefied gas pressure;    -   P_(a) is the sum of the fluid pressure forces acting on the        valve member from the auxiliary space side, including the vapor        pressure and static liquefied gas pressure;    -   P_(g) is the force of gravity acting on the valve member;    -   M_(ma) is the fluid momentum force acting on the valve member        caused by fluid moving through the valve from the auxiliary        space to the main storage space; and    -   M_(am) is the fluid momentum force acting on the valve member        caused by fluid moving through the valve from the auxiliary        space to the main storage space.

For a valve that is gravity biased in an open position, such as valves122, 122B, 122C, and 122D in respective FIGS. 1, 1A, 1B, 1C and 1D, thevalve member is urged to and remains in an open position, spaced apartfrom the valve seat when P_(m)<P_(a)+P_(g)+M_(am). If the valve memberis initially seated against the valve seat as shown in FIG. 1C, M_(am)is zero or substantially zero even if the valve member allows someleakage between itself and the valve seat. Once the valve member isspaced from the valve seat and cryogenic fluid is flowing through firstfluid passage 120 from auxiliary space 108 to main storage space 106,M_(am) can increase to a more significant value, adding to the forcesthat urged the valve member to the open position. The valve member isurged back to the seated position when P_(m)+M_(ma)>P_(a)+P_(g). Thiscan occur, for example, when main storage space 106 is being filled, thevapor pressure differential across partition 104 is reduced, and theincreasing level of liquefied gas in main storage space 106 causes fluidflow in first fluid passage 120 to reverse direction, whereby M_(ma)adds to the fluid forces urging the valve member towards the valve seat.When the valve member is pressed against the valve seat, M_(ma) is zero(or substantially zero if only a small amount of leakage occurs throughthe valve). It is worthwhile noting that in preferred embodiments, aperfectly fluid tight seal when the valve member is in the seatedposition is not essential. For example, the valve associated with firstfluid passage 120 will still function to achieve the intended resulteven if the valve allows a small amount of leakage between the valvemember and the valve seat, as long as the leakage is small enough topreserve the desired volume of vapor space during filling. That is, evena partially seated valve member or a valve member that is imperfectlysealed against the valve seat will still provide a constriction in thefluid passage preventing a significant amount of fluid from flowingthrough the fluid passage. Because the disclosed apparatus is tolerantof some leakage through the valves, a low degree of precision isacceptable for manufacturing specifications, which means that the costof manufacturing the valve can be kept low. A small amount of leakagecan actually help to equalize vapor pressure and the level of liquefiedgas in cryogen space 102, 402 after the re-filling process, duringnormal operation when holding and dispensing cryogenic fluid. Beingtolerant of some leakage also makes the design robust, meaning that itwill still operate acceptably even if the fluid seal is not perfect,which is important for an apparatus that comprises components that aredisposed inside the cryogen space where they are difficult, if notentirely impractical, to access for repair.

For a valve that has a valve member that is gravity biased in a seatedposition, such as valves 132E and 132F in respective FIGS. 1E and 1F,the valve member is urged to and remains in a seated position, pressedagainst the valve seat when P_(m)+M_(ma)<P_(a)+P_(g). If the valvemember is initially seated against the valve seat, M_(ma) is zero orsubstantially zero. Once the valve member is urged to a position whereit is spaced apart from the valve seat and fluid is flowing from mainstorage space 106 to auxiliary space 108 through second fluid passage130, M_(ma) increases to a significant value, adding to the forcesacting on the valve member to keep it in an open position. With valvemembers 134E and 134F biased by gravity in seated positions therespective valves act effectively like one-way valves since higher fluidpressures in auxiliary space 108 and fluid momentum forces associatedwith fluid flow through second fluid passage 130 from auxiliary space108 to main storage space 106 would both act on valve members 134E and134F to urge them to their respective seated positions.

As already mentioned in the description of these embodiments, when mainstorage space 106 is being vented, it is desirable for a higher vaporpressure to develop in auxiliary space 108 so that liquefied gas willflow from auxiliary space 108 into main storage space 106 though firstfluid passage 120. Accordingly, if valves 132E and 132F remain closedduring the venting of main storage space 106, this helps to maintain avapor pressure differential that promotes this objective.

Valves 132E and 132F open when P_(m)+M_(ma)>P_(a)+P_(g) and this occurswhen vapor pressure within main storage space 106 is higher than fluidpressure and gravitational forces acting on valve members 134E and 134F,respectively, allowing vapor in main storage space 106 to flow intoauxiliary space 108 through second fluid passage 130.

In the disclosed embodiments, the valve(s) that are associated with thefluid passages through the partition and disposed inside cryogen space102 operate to regulate flow between main storage space 106 andauxiliary space 108 and are actuated primarily by fluid forces. In someembodiments, additional forces such as gravitational forces can also beapplied to help bias the valve member in an open or closed position. Itis recognized that other biasing forces can be also be employed to thevalve member, such as spring forces or buoyancy forces. While theillustrated embodiments show a spherical valve member, persons familiarwith the technology involved here will understand that the valvemechanism can be of a different type, such as, for example, a hingedvalve member. The type of valve is not important as long as it can bemade with the requisite robustness, durability, and expected servicelife, because it is difficult if not entirely impractical to repair avalve that is disposed within the cryogen space. If consistent with thisrequirement, in other embodiments (not shown) the valve member can use aspring to bias it to an open or seated position. For example, referringnow to valve 132G shown in FIG. 1G, this valve could further include aspring disposed between valve member 134G and the open end in auxiliaryspace 108, whereby the spring would bias valve member 134G against valveseat 136G. In this example, the spring force could be added to theequation for describing its operation. There could also be embodimentsthat employ a different combination of forces acting on the valve memberin addition to the fluid forces, and for such embodiments, personsfamiliar with the technology involved here would understand how torevise the equations described herein to account for the additionalforces acting on the valve member.

In the following paragraphs, different operational scenarios will bedescribed to further elaborate on the preferred embodiments.

Venting prior to re-filling. There can be significant variability in thevapor pressure inside storage tanks that are brought to a fillingstation for re-filling. Some of the many different variables that caninfluence vapor pressure include ambient temperature, how long thecryogenic fluid has been held in the storage tank, and the initial vaporpressure after the last re-fill. It is common practice to vent acryogenic storage tank prior to re-filling, to reduce the vapor pressurewithin the cryogen space. Depending on the filling station's deliverypressure, vapor pressure in the cryogen space can slow the delivery ofcryogenic fluid into the storage tank. A problem with prior art designsthat have a vertical partition and an orifice restricting flow throughthe partition is that over time the level of liquefied gas inside thecryogen space can equalize in the two spaces separated by the partition.Consequently, when a partially emptied storage tank is brought to afilling station, there can be liquefied gas in the auxiliary space. Withprior art storage tanks, during venting and re-filling, the liquefiedgas in the auxiliary space can not drain quickly enough into the mainstorage space through the orifice, so after the main storage space isfilled, the reserved vapor space is smaller than desired becauseliquefied gas still occupies some of the auxiliary space. With referenceto the storage tank of FIG. 1 as an example, the present design solvesthis problem. During the venting process, vapor vents from main storagespace 106 faster than it can vent from auxiliary space 108 because ofthe constricted flow area through second fluid passage 130. This causesa pressure differential to develop between auxiliary space 108 and mainstorage space 106, with the higher pressure in auxiliary space 108pushing liquefied gas from the bottom of auxiliary space 108 throughfirst fluid passage 120 and open valve 122. First fluid passage 120 hasa larger cross sectional area compared to second fluid passage 130 sothat the liquefied gas can be drained quickly from auxiliary space 108.As liquefied gas is removed from auxiliary space 108 and as some vaporis vented through second fluid passage 130, the pressure differentialbetween auxiliary space 108 and main storage space 106 graduallydeclines, until fluid pressures equalize and cryogenic fluid ceases toflow from auxiliary space 108 to main storage space 106.

With the understanding that storage tanks can be brought for re-fillingin different states, some with higher vapor pressures, some with higherlevels of residual liquefied gas, and so on, FIG. 2A is a plot thatshows by way of one example, how a preferred embodiment of the disclosedstorage tank operates to drain liquefied gas from the auxiliary spacewhile the main storage space is being vented. This illustrative exampleis representative of how the disclosed storage tank can operate underdifferent conditions. FIG. 2A is a plot of vapor pressure (with thescale on the left hand axis) against time in seconds overlaid with thelevel of liquefied gas inside a storage tank expressed as a percentagefrom full (with this scale on the right hand axis) plotted against thesame time scale. In this example, the plotted data is calculated basedupon the liquefied gas being methane and a storage tank that isgenerally cylindrical in shape with a horizontal longitudinal axis, likethe storage tank shown in FIGS. 1 and the fluid passages through thepartition as shown in FIG. 1D, with the following characteristics: acryogen space with a volumetric capacity of about 75 U.S. gallons (about0.28 cubic meters); a fill conduit 110, which can also be used to ventcryogen space 102, with an internal diameter of about ¾ inch (about 19mm); a first fluid passage with an internal diameter that is about 0.3inch (about 7.6 mm); and, a second fluid passage that is a tube with aninternal diameter that is about 3/16 inch (about 4.75 mm). In thisexample, before beginning to vent cryogen space 102 through fill conduit110, the level of liquefied gas on both sides of the partition isequalized, initially at 50% (half empty), and the initial vapor pressurein the cryogen space is also equalized, at 170 pounds per square inch(psi; about 1.2 MPa). Line 210 is the plot of the vapor pressure withinthe main storage space and dashed line 212 is the plot of the vaporpressure within the auxiliary space, both plotted against time. Theventing of cryogen space 102 begins at 0 seconds on the time scale. Line214 is the plot of the level of liquefied gas within the main storagespace and dashed line 216 is the plot of the level of the liquefied gaswithin the auxiliary space, both plotted against the same time scale asthe vapor pressure data. FIG. 2A shows, surprisingly, that only a smallpressure differential between the auxiliary space and the main storagespace is all that is needed to drain the liquefied gas from theauxiliary space in a short period of time, showing, in this example, thelevel of liquefied gas dropping from 50% to about 3% in less than 220seconds, which is less time that it takes to drop the vapor pressurefrom 170 psi (about 1.2 MPa) to 120 psi (about 0.8 MPa). When vapor isvented from main storage space 106, vapor cannot be vented fromauxiliary space 108 as quickly, causing a pressure differential todevelop. Near the end of the venting process the vapor pressure oneither side of the partition again equalizes as vapor pressure withinauxiliary space 108 eventually catches up with the reduced vaporpressure within main storage space 106. Over the course of this ventingprocedure, in this example, a maximum pressure differential of about 0.7psi (about 5 kPa) develops. Whereas prior art storage tanks restrictedflow between the partitioned spaces, which can inhibit drainingliquefied gas from the auxiliary space, the disclosed apparatus iseffective in draining even high levels of liquefied gas from theauxiliary space because first fluid passage 120 is sized large enough sothat it does not overly restrict the flow of liquefied gas fromauxiliary space 108 to main storage space 106. Once the liquefied gas isdrained from auxiliary space 108 and vapor pressures equalizes, valve122 is operative to restrict significant amounts of liquefied gas fromflowing back into auxiliary space 108 even after vapor pressuresequalize.

Re-Filling. In a preferred method, to begin re-filling a storage tankwith cryogenic fluid, cryogenic fluid from a filling station isintroduced through fill conduit 110 into main storage space 106 afterthe vapor pressure within main storage space 106 has been reduced tobelow a predetermined pressure threshold. As described in the previousparagraph, in a preferred method liquefied gas is drained from theauxiliary space during the venting process, but in a worst-casescenario, there can still be liquefied gas in the auxiliary space whenre-filling commences. For example, a storage tank brought for re-fillingcan initially have a low vapor pressure whereby venting is notnecessary, but equalized levels of liquefied gas within the cryogenspace mean that there can be a significant amount of liquefied gaswithin the auxiliary space when re-filling commences. Even with suchscenarios, the presently disclosed storage tank is operative to draincryogenic fluid from auxiliary space 108 to main storage space 106,during re-filling as long as the vapor pressure within auxiliary space108 is higher than vapor pressure within main storage space 106. Thevapor pressure within main storage tank 106 can decline during fillingbecause the new cryogenic fluid being introduced through fill conduit110 further cools and condenses the vapor within main storage space 106.A pressure differential can develop because unlike the vapor within mainstorage space 106, partition 104 prevents the vapor within auxiliaryspace 108 from coming into direct contact with the cryogenic fluid beingintroduced through fill conduit 110. FIG. 2B is a plot that shows by wayof one example, how the disclosed storage tank is operative to drainliquefied gas from auxiliary space 108 while main storage space 106 isbeing re-filled with cryogenic fluid. Like FIG. 2A, the illustrativeexample of FIG. 2B is representative of how the disclosed storage tankcan operate under different conditions. FIG. 2B plots the sameparameters as FIG. 2A and assumes a storage tank with the samecharacteristics, with the difference being the starting conditions andthe time span corresponding to when the storage tank is being re-filledwith cryogenic fluid rather than when it is being vented. In thisexample, the fill station has a delivery pressure of 50 psi (about 0.3MPa). At 0 seconds on the time scale, the vapor pressure is 120 psi(about 0.8 MPa) inside both main storage space 106 and auxiliary space108 and the level of liquefied gas on both sides of the partition isequalized, again initially at 50% (half empty). Line 220 is the plot ofthe vapor pressure within the main storage space and dashed line 222 isthe plot of the vapor pressure within the auxiliary space, both plottedagainst time. The re-filling of cryogen space 102 begins at 0 seconds onthe time scale. Line 224 is the plot of the level of liquefied gaswithin the main storage space and dashed line 226 is the plot of thelevel of the liquefied gas within the auxiliary space, both plottedagainst the same time scale as the vapor pressure data. Once again, withthe disclosed apparatus, FIG. 2B shows, again surprisingly, that only asmall pressure differential between the auxiliary space and the mainstorage space is all that is needed to drain the liquefied gas from theauxiliary space in a short period of time, showing, in this example, thelevel of liquefied gas dropping from 50% to about 15% in less than 100seconds, which is about the same time that it takes to re-fill mainstorage space 106 with cryogenic fluid. Near the end of the re-fillingprocess the rate at which vapor pressure declines in main storage space106 also declines (as shown by the shallower slope of line 220 on theright hand side of FIG. 2B). After the introduction of cryogenic fluidis stopped, the vapor pressure on either side of partition 104 equalizeswhen vapor pressure within auxiliary space 108 eventually catches upwith the reduced vapor pressure within main storage space 106. In thisexample, over the course of the re-filling procedure, a maximum pressuredifferential of about 3.3 psi (about 23 kPa) develops, which is evenhigher than the pressure differential that developed in the ventingprocedure example. The applicant found that with prior art storagetanks, even pressure differentials of this order of magnitude were notsufficient under some conditions to drain the liquefied gas from theauxiliary space to the desired level because the size of orificeemployed by prior art designs restricted flow between the partitionedspaces.

The examples illustrated by FIGS. 2A and 2B, both show that thedisclosed apparatus is effective in draining even high levels ofliquefied gas from the auxiliary space because first fluid passage 120is sized large enough so that it does not overly restrict the flow ofliquefied gas from auxiliary space 108 to main storage space 106. Withthe disclosed storage tank, if vapor pressure equalizes or the fluidpressure differential reverses with the rising static fluid pressure inmain storage space 106 and the declining vapor pressure in auxiliaryspace 108, flow of liquefied gas from auxiliary space 108 can eventuallystop. Once fluid pressure within main storage space 106 is higher thanfluid pressure within auxiliary space 108, then valve 122 closes andfluid flow through first fluid passage 120 is blocked. In this example,valve 122 closes when fluid flow through first fluid passage 120 in thedirection from main storage space 106 to auxiliary space 108 exceedsabout 0.3 ounces/second (about 9 grams/second). Accordingly, cryogenicfluid does not flow back into auxiliary space 108 through first fluidpassage 120 in significant amount because valve 122 is operative underthe influence of fluid forces to restrict cryogenic fluid from flowingfrom main storage space 106 to auxiliary space 108.

The venting and re-filling scenarios described in the previousparagraphs demonstrate how the disclosed storage tank can operate duringthe venting or re-filling procedure to reserve a low-pressure vaporspace from which liquefied gas has been evacuated, thereby preserving aspace that can act as an ullage space into which cryogenic fluid canlater expand. Because second fluid passage 130 is located near the topof partition 104, during re-filling, liquefied gas does not have anopportunity to flow through it until main storage space 106 is almostfull. Even then, because second fluid passage has a constricted flowarea, the amount of liquefied gas that does flow through it intoauxiliary space 108 is negligible. The constricted flow area of firstand second fluid passages 120, 130, especially when valve 122 is closed,cause a sudden increase in back-pressure when main storage space 106 isbeing re-filled and main storage space 106 is filled with liquefied gas.This sudden increase in back-pressure can be detected by the fillingstation so that it automatically stops delivering cryogenic fluid toprevent over-filling.

Holding and dispensing cryogenic fluid during normal operation. Aftermain storage space 106 has been filled with liquefied gas, if heat leaksinto cryogen space 102 and causes liquefied gas to be warmed andexpanded, the compressed vapor can flow into auxiliary space 108 throughsecond fluid passage 130, preventing a sudden increase in vapor pressurethat would otherwise occur if there were no space into which the vaporcould flow. Because second fluid passage 130 allows vapor to move freelythrough partition 104, and because partition 104 is thermallyconductive, over time, pressure and temperature will equalize withincryogen space 102. Since valve 122 is gravity biased open, if the fluidpressure on the opposite ends of first fluid passage 120 are the same,liquefied gas can seep from main storage space 106 to auxiliary space108 through valve 122 as long as the fluid momentum forces associatedwith such flow are less than the gravity force that urges valve member124 to an open position. Over time, the level of liquefied gas in mainstorage space 106 will be substantially the same as the level ofliquefied gas in auxiliary space 108 but this will not inhibit operationof the storage tank because the objective of preserving a desired volumeof vapor space has already been achieved, and once the storage tank isre-filled, the location of the vapor space within cryogen space 102 isirrelevant. That is, as long as the desired volume of vapor space ispreserved, it does not matter if the vapor occupies the upper region ofcryogen space 102 on both sides of partition 104. First fluid passage120 and valve 122 are sized so that liquefied gas can flow there throughat a rate that is high enough to allow the level of liquefied gas onopposite sides of partition 104 to stay substantially equal even whencryogenic fluid is being dispensed at the designed maximum flow rate,and to allow liquefied gas within auxiliary space 108 to be evacuatedthere from when main storage space 106 is being vented prior tore-filling. This is an improvement over prior art tanks that employed asingle opening in the partition to serve two functions which haveopposite requirements. That is, a single fluid passage should to besmall enough to restrict flow into the auxiliary space duringre-filling, but this is disadvantageous when one is trying to emptyliquefied gas from the auxiliary space when the storage tank is beingvented or re-filled, or when liquefied gas is being dispensed at themaximum rate. By having two fluid passages, one is sized with aconstriction to restrict flow of liquefied gas during re-filling whilebeing large enough to allow vapor expansion and equalization duringnormal operation, and the other fluid passage, is equipped with a valveto prevent a significant amount of liquefied gas from flowing into theauxiliary space during re-filling, while being sized large enough toallow higher flow rates from the auxiliary space to the main storagespace when the storage tank is being vented or re-filled, or whencryogenic fluid is being dispensed at a high rate.

The embodiment of FIG. 3, is like the embodiment of FIG. 1 with likefeatures bearing like reference numbers. The main difference is thatstorage tank 300 comprises pump 350, which is disposed with a suctioninlet within main storage space 106 to remove cryogenic fluid there fromand deliver it to delivery conduit 352. Persons familiar with thetechnology involved here will understand that the first and/or secondfluid passages shown in FIG. 3 could be replaced with one of therespective first or second fluid passages shown in FIGS. 1B through 1G.

FIG. 4 illustrates a schematic view of storage tank 400 whereinpartition 404 is more horizontally oriented to divide cryogen space 402into main storage space 406 and auxiliary space 408. While it can bemore difficult and more expensive to manufacture a partition with ahorizontal orientation, this embodiment has some functional advantagesfor draining liquefied gas from auxiliary space 408. Further, thisembodiment shows that other configurations are contemplated within thescope of the claimed design. Different orientations for the partitioncan be used. Like the other embodiments, cryogenic fluid can beintroduced into cryogen space 402 through fill conduit 110, which hasoutlet 111 that opens into main storage space 406. First fluid passage420 is disposed with an opening at a low point of auxiliary space 408 tofacilitate the gravity assisted flow of liquefied gas from auxiliaryspace 408 into main storage 406. Valve 422 comprises valve member 424which can be gravity biased in an open position. Valve member 424 can bemade to be buoyant in the liquefied gas so that buoyancy forces can alsoact on valve member 424 in addition to fluid momentum forces to urge ittowards valve seat 426 when the level of liquefied gas in main storagespace 406 is high enough. Screen 428 permits cryogenic fluid to enterand exit from first fluid passage 420, while preventing valve member 424from falling out. Because auxiliary space 408 is in the upper region ofcryogen space 402, unlike embodiments with a vertically orientedpartition, liquefied gas need not enter auxiliary space to maintain theliquefied gas at a substantially constant level across the bottom ofcryogen space 402. Accordingly, valve 422 can be a one-way valve thatallows cryogenic fluid to flow through first fluid passage only fromauxiliary space 408 to main storage space 406. Second fluid passage 430comprises an orifice connecting an upper region of main storage space406 with an upper region of auxiliary space 408. The orifice is sized toconstrict the flow of liquefied gas from main storage space 406 toauxiliary space 408. During filling, size and location of the orificereduces the flow of liquefied gas from main storage space 406 to aninsignificant amount since such flow is limited to the time when thelevel of the liquefied gas in main storage space 406 has risen to thelevel of the orifice, near the end of the filling process. Second fluidpassage 430 helps to equalize vapor pressure throughout cryogen space402 and to prevent valve member 424 from being stuck in the closedposition if valve member 424 makes a fluid tight seal against associatedvalve seat 426. Like other embodiments, first fluid passage 420 can besized to accommodate higher flow rates from auxiliary space 408 to mainstorage space 406 because valve 422 prevents liquefied gas from flowingin the reverse direction during filling. During normal operation whenstorage tank 400 is holding and dispensing liquefied gas, second fluidpassage 430 permits vapor to flow from main storage space 406 intoauxiliary space 408 to slow vapor pressure increases caused by heat leakand expansion of the liquefied gas within cryogen space 402. Whenstorage tank 400 is brought to a filling station to be re-filled, withthe configuration of this embodiment, the level of liquefied gas in mainspace 406 will normally be below the low point of auxiliary space 408,so all of the liquefied gas that may have been held in auxiliary space408 will be drained already into main storage space 406 through valve422. Since there will not typically be liquefied gas in auxiliary space408, one difference is that it will not be important to establish apressure differential like in the other embodiments to push liquefiedgas from the auxiliary space to the main storage space. With storagetank 400, during venting, prior to filling, the larger flow capacity offirst fluid passage 420 together with open second fluid passage 430 willboth permit vapor to be vented from auxiliary space 408 more quicklythan prior art designs, which restrict fluid flow, and this isadvantageous because it is desirable for the vapor pressure to bereduced to a lower pressure, to increase the capacity of the ullagespace to accept more vapor and expanded liquefied gas before there is aneed to vent vapor through the pressure relief valve. If the fillstation's delivery pressure happens to be higher than the vapor pressurein auxiliary space 408, if the pressure differential is high enough tocause valve member 424 to move towards valve seat 426, this can preservethe lower vapor pressure in auxiliary space 408 during re-filling.

While particular elements, embodiments and applications of the presentinvention have been shown and described, it will be understood, ofcourse, that the invention is not limited thereto since modificationscan be made by those skilled in the art without departing from the scopeof the present disclosure, particularly in light of the foregoingteachings.

1. A method of holding a cryogenic fluid comprising liquefied gas andvapor in a thermally insulated cryogen space defined by a storage tank,wherein said cryogen space is partitioned so that it is divided into amain storage space and an auxiliary space, said method comprising: (a)establishing a vapor-filled ullage space when said main storage space isbeing vented and re-filled with cryogenic fluid, by draining liquefiedgas from said auxiliary space to said main storage space through a firstfluid passage provided through said partition, wherein said first fluidpassage is sized to allow said liquefied gas to flow there through underthe influence of a vapor pressure differential permitted by saidpartition, said liquefied gas draining at a rate that allows saidauxiliary space to be drained of liquefied gas to establish saidvapor-filled ullage space with a volume that is at least as large as apredetermined ullage volume in less than the time needed to vent andre-fill said main storage space with cryogenic fluid; (b) preservingsaid vapor-filled ullage space by restricting flow of liquefied gasthrough said first fluid passage from said main storage space into saidauxiliary space when said cryogen space is being re-filled withcryogenic fluid; (c) restricting cryogenic fluid flow through a secondfluid passage provided through said partition, so that said vaporpressure differential can develop across said partition when said mainstorage space is being vented and re-filled with cryogenic fluid, andwhereby a detectable back pressure occurs when said main storage spaceis filled with cryogenic fluid when cryogenic fluid is being introducedinto said main storage space; and (d) detecting when said storage tankis full and stopping re-filling when said storage tank is full.
 2. Themethod of claim 1 wherein the step of restricting flow through saidfirst fluid passage comprises regulating flow there through with a valvethat comprises a valve member that is actuatable between an open and aseated position by fluid forces within said cryogen space, whereby fluidflow is restricted by said valve when said valve member is in saidseated position.
 3. The method of claim 2 wherein said fluid forceswithin said cryogen space that act on said valve member comprise vaporpressure and static liquefied gas pressure, and dynamic fluid momentumforces generated by said cryogenic fluid when it is flowing through saidvalve.
 4. The method of claim 2 further comprising orienting said valvewithin said cryogen space so that gravitational forces act on said valvemember to bias it towards an open position.
 5. The method of claim 2wherein said valve member is urged towards said seated position whenfluid pressure acting on said valve member from fluid within said mainstorage space is higher than fluid pressure acting on said valve memberfrom fluid within said auxiliary space by a predetermined margin, andsaid valve member is otherwise urged to said open position.
 6. Themethod of claim 2 wherein when filling said main storage space with saidcryogenic fluid, said method comprises introducing said cryogenic fluidat a flow rate that causes cryogenic fluid level within said mainstorage to increase at a faster rate than cryogenic fluid level can risewithin said auxiliary space, whereby static fluid pressure and fluidmomentum pressure acting on said valve member urge it towards saidseated position.
 7. The method of claim 1 wherein the step of detectingwhen said storage tank is full comprises processing a signal from alevel sensor to determine the level of liquefied gas within said mainstorage space.
 8. The method of claim 1 wherein flow through said secondfluid passage is restricted by making the cross-sectional flow area ofsaid second fluid passage smaller than that of a fill conduit throughwhich cryogenic fluid is introduced into said main storage space whenre-filling said storage tank.
 9. The method of claim 4 furthercomprising venting vapor from said main storage space through said fillconduit prior to re-filling said storage tank with cryogenic fluid. 10.The method of claim 1 further comprising venting vapor from said mainstorage space through a vent pipe prior to re-filling said storage tankwith cryogenic fluid.
 11. The method of claim 1 wherein when re-fillingsaid main storage space, said cryogenic fluid is not introduced intosaid cryogen space until vapor pressure within said main storage spaceis below a predetermined value.
 12. The method of claim 2 furthercomprising regulating fluid flow through said second fluid passage witha second valve comprising a valve member that is actuatable by fluidforces within said cryogen space, whereby said second valve opens toallow cryogenic fluid to flow from said main storage space to saidauxiliary space when vapor pressure in said main storage space is higherthan vapor pressure within said auxiliary space by a predeterminedamount.
 13. The method of claim 12 wherein said second valve is aone-way valve, and said method further comprises allowing fluid to flowthrough said second valve only from said main storage space to saidauxiliary storage space.
 14. The method of claim 1 further comprisingtransferring said cryogenic fluid between a lower region of saidauxiliary space and an upper region of said main storage space, whensaid cryogenic fluid is flowing through said second fluid passage. 15.The method of claim 1 further comprising dispensing said cryogenic fluidfrom said main storage space through an outlet conduit.
 16. The methodof claim 1 further comprising pumping said cryogenic fluid from saidmain storage space with a pump that has an inlet disposed within saidcryogen space.